1. Technical Field
The present invention relates to an insulation resistance detecting apparatus for detecting a reduction when an insulation resistance of a load circuit is reduced due to electric leakage or the like.
2. Background Art
For example, in a motor drive circuit (hereinbelow referred to as a “load circuit”) provided in an electric vehicle and a hybrid vehicle, a high-voltage direct current power source is provided, power outputted from the direct current power source is converted to three-phase alternating current power, and the three-phase alternating current power is supplied to the motor to thereby drive the motor.
In the load circuit described above, an insulation resistance detecting apparatus (see, e.g., JP-A-2007-163141) is provided in order to prevent electric leakage and, when the insulation resistance of the load circuit is reduced due to the occurrence of the electric leakage, an alarm is issued to immediately notify a user of the occurrence of the electric leakage.
FIG. 6 is a circuit diagram showing a structure when an insulation resistance detecting apparatus 50 is connected to a load circuit 10 in a conventional case. As shown in FIG. 6, the insulation resistance detecting apparatus 50 includes a coupling capacitor C1, a rectangular wave generating circuit 51, a self-diagnosis circuit 52, a filter 53, a waveform shaping circuit 54, and a microcomputer 55. One end of the coupling capacitor C1 is connected to a point P1 as one output terminal of a direct current power source B provided in the load circuit 10, while the other end thereof is connected to a point P2 as an output terminal of the rectangular wave generating circuit 51.
When a rectangular wave is outputted from the rectangular wave generating circuit 51, the waveform of the rectangular wave is transformed into a sinusoidal waveform in the filter 53, and is further shaped into a waveform in which a region in the vicinity of a peak point is enlarged in the waveform shaping circuit 54. A wave height value of a waveform of a signal outputted from the waveform shaping circuit 54 (hereinbelow referred to as a reference signal V1) changes in accordance with the magnitude of an insulation resistance Ri. Specifically, when the insulation resistance Ri is large (normal case), the voltage at the point P2 is high so that the wave height value of the reference signal V1 is high. On the other hand, when the insulation resistance Ri is reduced due to the occurrence of the electric leakage in the load circuit 10, the voltage at the point P2 is reduced so that the wave height value of the reference signal V1 is reduced.
A curve L1 shown in FIG. 7 is a characteristic view showing a relationship between the insulation resistance Ri and the wave height value of the reference signal V1, and the microcomputer 55 determines the insulation resistance Ri of the load circuit 10 on the basis of the reference signal V1 outputted from the waveform shaping circuit 54. Herein, as seen from the curve L1, when the electric leakage does not occur in the load circuit 10, and the insulation resistance Ri of the load circuit 10 is large, the wave height value of the reference signal V1 is large; when the insulation resistance Ri is reduced, the wave height value of the reference signal V1 is sharply reduced; and when the insulation resistance Ri reaches the vicinity of a zero point, the wave height value of the reference signal V1 becomes almost zero.
Subsequently, when the wave height value of the reference signal V1 is reduced to be less than a predetermined threshold value, the microcomputer 55 can determine that the insulation resistance of the load circuit 10 is reduced, and issue an alarm or the like to notify the user of the reduction.
In the conventional insulation resistance detecting apparatus 50 described above, when a short circuit trouble occurs in the load circuit 10, and the insulation resistance Ri is reduced to be in the vicinity of zero, the wave height value becomes almost zero. On the other hand, when the short circuit fault occurs in the insulation resistance detecting apparatus 50 itself, the wave height value detected by the microcomputer 55 similarly becomes almost zero. That is, when the microcomputer 55 detects that the wave height value is zero, there is a case where it is not possible to discern whether the short circuit trouble is occurring in the load circuit 10 or the short circuit fault is occurring in the insulation resistance detecting apparatus 50 itself.
Consequently, in the conventional insulation resistance detecting apparatus 50, when an ignition is turned on, the self-diagnosis circuit 52 is operated to perform a process for determining whether or not the fault is occurring in the entire apparatus. Specifically, when the ignition is turned on, the microcomputer 55 shown in FIG. 6 outputs a forced electric leakage signal to turn on the self-diagnosis circuit 52, whereby the voltage at the point P2 is reduced to a certain level, and the situation in which the electric leakage is occurring in the load circuit 10 is artificially produced. Subsequently, when the microcomputer 55 detects a reduction in the insulation resistance Ri, it is determined that the insulation resistance detecting apparatus 50 is normally operated.
However, the self-diagnosis circuit 52 mentioned above has a drawback that the fault determination is performed only when the ignition is turned on, but the fault can not be detected in a case where the fault occurs in the insulation resistance detecting apparatus 50 after the ignition is turned on.